The invention relates to an X-ray image intensifier tube, comprising an entrance screen provided with an aluminum substrate which supports a scintillator for transforming X-rays, which reach the scintillator through the substrate, into visible or nearly visible light radiation which is converted, by way of a photocathode, into a flux of electrons to produce a visible image on an exit screen via electron-optical means.
A device of this kind is disclosed in the document FR No. 2 515 423. It describes an entrance secreen which is suitable for use in a brightness intensifier tube offering an improved resolution. To this end, the columnar cesium iodide crystals constituting the scintillator stem from impurity particles on the surface of the aluminum substrate. A secondary effect of these surface impurities is that they stimulate the absorption of the light emitted in the direction of the substrate. Because the light-guided effect of the columnar crytals is not perfect, this absorption of the light by the impurity particles improves the contrast of the image formed. These impurity particles form seeds for growing the columnar crystals. Therefore, they must be islands disseminated across the surface of the aluminum substrate, which islands are made formed by an appropriate chemical process. The light emitted by the scintillator in the direction of the substrate is thus only secondarily absorbed by these impurity particles whose presence and nature are random.
The problem to be solved, therefore, consists in the construction of a tube comprising an entrance window having a high resolution for the entire area of the image formed. The performance of the tube must be reproducible and reliable.